Physical Effects of Variable Fluid Properties on Laminar Gas Microconvective Flow

The physical effects of variable fluid properties on heat transfer and frictional flow characteristics of laminar gas microconvective flow are investigated in this research. The fully developed flow through a microtube is studied numerically by using 2D continuum‐based governing equations. The physical effects induced due to variations in gas density with pressure and temperature, and gas viscosity, thermal conductivity, and specific heat with temperature are analyzed. Numerical results reveal that the heat transfer and frictional flow characteristics of laminar gas microflow are drastically affected by these physical effects. Hence, this research suggests that these physical effects need to be well considered in the applications of laminar gas microconvection based on large temperature gradients, for example, the design of microchannel heat sinks, and the flow cannot be generally considered as a constant property flow, as in conventional channels.     

Slip Flow of Casson Rheological Fluid Under Variable Thermal Conductivity with Radiation Effects

This paper investigates the radiation and chemical reaction effects on Casson non‐Newtonian fluid towards a porous stretching surface in the presence of thermal and hydrodynamic slip conditions. The governing boundary layer conservation equations are normalized into nonsimilar form using similarity transformations. A numerical approach is applied to the resultant equations. The behavior of the velocity, temperature, concentration, as well as the skin friction coefficient, Nusselt number, and Sherwood number for various governing physical are discussed. Increasing the radiation parameter decreases the temperature. An increase in the rheological parameter (Casson parameter) induces an elevation in the skin friction coefficient, the heat and mass transfer rates. The larger the β values the closer the fluid is in behavior to a Newtonian fluid and further departs from plastic flow. Temperature of the fluid was found to decrease with increasing values of the Casson rheological parameter. The most important non‐Newtonian fluid possessing a yield value is the rheological Casson fluid, which finds significant applications in polymer processing industries, biomechanics, and chocolate food processing.     

Effects of High-Order Slip/Jump,Thermal Creep,and Variable Thermophysical Properties on Natural Convection in Microchannels With Constant Wall Heat Fluxes

Natural convection gaseous slip flows in open-ended vertical parallel-plate microchannels with symmetric wall heat fluxes are numerically investigated. A second-order model, including thermal creep effects, is considered for velocity slip and temperature jump boundary conditions with variable thermophysical properties. Simulations are performed for wide range of Rayleigh numbers from 5 × 10^{? 6} to 5 × 10^{? 3} in the continuum to slip flow regime. The developing and fully developed solutions are examined by solving the Navier–Stokes and energy equations using a control volume technique. It is found that the second-order effects reduce the temperature jump and the slip velocity, whereas thermal creep strongly increases the slip velocity in both developing and fully developed regions. Moreover, the rarefaction effects increase the flow and heat transfer rates considerably, while decreasing the maximum gas temperature and friction coefficient as compared to the continuum limit. It was also shown that the axial temperature variations of the gas layer adjacent to the wall in the modeling of the thermal creep are of paramount importance and neglecting these variations, which is common in literature, leads to unphysical velocity and temperature distributions.     

LES of Stably Stratified Flow with Varying Thermophysical Properties

Large eddy simulations of stably stratified, turbulent channel flow subjected to a large temperature gradient have been performed by considering a broad spectrum of stratification. Due to a large thermal gradient across the channel, temperature-dependent fluid properties like viscosity (µ), density (ρ), and thermal conductivity (κ) are considered as variables. With increased stratification, a cluster of laminar patches appears in the near-wall region, and turbulent momentum and buoyancy fluxes are suppressed drastically in the core of the channel due to the formation of internal gravity waves. Variable viscosity results in flow relaminarization on the hot side of the channel (where viscosity is higher). Density tends to stablize the flow by blocking the upward movement of thermal plumes, while thermal conductivity pays the toll for viscosity at high Reynolds number. A mechanistic model for wall heat transfer is developed for buoyancy-effected flows. We have observed qualitatively and quantitatively the pronounced modifications in turbulent structure and flow statistics.     

连铸中间罐内流场和温度场数值模拟研究

以本钢连铸中间罐为研究对象,通过建立罐内钢液的流动的数学模型,计算出六流T型中间罐内钢液的流动状况和温度分布情况,其结果表明:在罐内中间流与边流之间钢液流速和温度分布是存在差异的.因此,确定了有利于发挥中间罐冶金功能的控流方案,为连铸稳定生产获得合格坯提供了理论依据.     

四流连铸中间包内流体流动行为的物理模拟

以某企业四流T型中间包为研究对象,建立模型与原型相识比为0.25的物理模型,并通过物理模拟方法系统研究了导流隔墙上的导流孔结构及湍流控制器结构对中间包内流体流动行为的影响。实验结果表明:优化导流孔结构及改变湍流控制器后,流体平均停留时间增加,相对增加率为21.1%;死区体积分率减小,相对减小率为32.4%;一流和二流的流动一致性增强,两流平均停留时间标准差仅为3 s。     

Recent Studies on Fluid Flow and Heat Transfer in Thermal Microdevices

Control and measurement of fluid flow and heat transfer in microdevices is of great importance to the development and application of MEMS and Bio-MEMS such as thermal inkjet printer heads, microchemical reactors, and PCR. Thus, the detailed flow behavior, in particular the two-phase flow in microdevices, has attracted much attention in recent years. Several types of thermal micropumps have been developed, although there is still room for further development. Various techniques for measuring the temperature, which are applicable to microscale devices, have also been proposed. As the cooling problem in microdevices becomes increasingly significant, a prospective view on integrated heat and mass transfer is quite necessary. Thus, in this work, some issues and future prospects for fluid dynamics and heat transfer of thermal microdevices are presented and discussed, in terms of thermocapillary pump, temperature measurement in microdevices, and flow near an evaporating meniscus.     

Convective Heat Transfer of an Air Bubble in Water With Variable Thermophysical Properties

A numerical study of convective heat transfer of an air bubble in water with variable thermophysical properties is considered. Two-dimensional simulations of multifluid flows with heat transfer include the Navier–Stokes, energy, and volume of fluid (VOF) advection equations. The solver computes the flow field and temperature by solving the systems of Navier–Stokes equations and the energy equation using the finite–volume method with the SIMPLE algorithm and tracks the position of interface between two fluids with different fluid properties by the VOF method with piecewise linear interface construction technique. Empirical correlations in terms of temperature for thermophysical properties are considered in the simulations. The convective heat transfer model is assessed with a benchmark problem of cooling of water and compared with previous literature data showing good agreement. Finally, a numerical study of the effect of the bubble diameter in the range from 2 mm to 3 mm on heat transfer is performed.     

纳米粒子与石蜡乳状液复合多相功能热流体的制备与性能

以石蜡乳状液为分散介质,纳米Cu粒子为导热介质,采用相转化乳化法制备了纳米Cu/石蜡复合相变乳状液,研究了纳米Cu粒子对复合相变乳状液的稳定性、流变性、导热性和热循环稳定性的影响。研究结果表明：纳米Cu/石蜡乳状液的密度、粘度和导热系数均随纳米Cu含量的增大而增大,密度变化不太明显,而导热系数则明显提高,当纳米Cu粒子含量为0.05 wt%时,复合相变乳状液的导热系数比纯石蜡乳状液提高了161.96%。由于纳米Cu粒子的添加显著增加了Cu/石蜡乳状液的导热性能,Cu/石蜡乳状液在固−液相变热循环过程中的温度平台表现不明显,但其热循环稳定性很好。     

Investigation of Excessive Material on Insulating Properties Using Different Heat Transfer Fluid for Thermal Energy Storage Development

Though TES （thermal energy storage） is developed hugely in most of the solar power generation plants, it is less growth in implementing a modular type of TES in a solar plant, e.g., solar dish/stifling engine application. The main issue in designing the TES system is its thermal capacity of storage materials, e.g., insulator. This study is focusing on the potential waste material as an insulator for thermal energy storage applications. The insulator usage is to reduce the heat transfer between two mediums and the capability is measured by its resistance to heat flow. It is needed to obtain optimal materials to energy conversion at the same time reduce the waste generation. Therefore, a small-scale experimental testing of natural cooling process of an insulated tank within a confined room without any forced cooling system, e.g., fan. The testing is repeated by changing the insulator using the potential waste material from natural and industrial waste and also by changing the HTF （heat transfer fluid）. The analysis is performed on the relationship between heat loss and the reserved period by the insulator. The results indicate the percentage of period of the insulated tank withstands the heat compared to non-insulated tank, e.g., cotton reserved the period of 14% more than non-insulated tank to withstand the heat transfer of cooking oil to the surrounding. The paper finally justifies the most potential waste material as an insulator in different heat transfer fluids.     

Numerical Modeling of Fluid Flow and Thermal Behavior of Different Nanofluids on a Rotating Disk

The thermal and velocity profiles of various nanofluid systems on a rotating disk are simulated. Finite difference method, the orthogonal collocation method, and the differential quadrature method (DQM) of numerical approaches are used to solve the governing equations and are compared to determine the faster and more accurate solution procedure. Five nanoparticles Al, Al₂O₃, Cu, CuO, and TiO₂ solved in three base fluids water, ethylene glycol, and engine oil are considered to be used on the disk at different volume fractions. A new general algorithm is presented for solving equations of a rotating‐disk problem quickly and accurately and it is found that the DQM method is the best approach for this numerical simulation. Heat transfer performance of a rotating disk would be much better enhanced with water based Al nanofluid. A wide range of results for different base–fluid combinations with nanoparticles is presented with untransformed 3D results and effects of the variation of different parameters provides comprehensive insight and prevents inaccurate deductions.     

Buoyancy- and Thermocapillary-Induced Convection of Cold Water in an Open Enclosure with Variable Fluid Properties

The aim of the present study is to investigate the buoyancy- and thermocapillary-induced convection of water near its density maximum in an enclosure with temperature-dependent properties. The viscosity and thermal conductivity of the water are varied with reference temperature. The governing equations are solved by the finite-volume method. The results are discussed for various values of reference temperature parameter, density inversion parameter, and Rayleigh and Marangoni numbers. It is observed that the temperature of maximum density leaves strong effects on flow and heat transfer due to the formation of bi-cellular structure and reduction of the average heat transfer. It is also observed that convection heat transfer is enhanced by thermocapillary force when buoyancy force is weaken.     

Comparison of Fluid Flow and Thermal Characteristics of Plate-Fin and Pin-Fin Heat Sinks Subject to a Parallel Flow

In this study, we compare the thermal performances of the two types of heat sinks most commonly used in the electronic equipment cooling: plate-fin and pin-fin heat sinks. In order to obtain the fluid flow and thermal characteristics of heat sinks, an experimental investigation is conducted. Based on the experimental results of the present study and the available data from the existing literature, the correlations of the friction factor and the Nusselt number are suggested for each type of heat sink. Correlations for the pin-fin heat sinks are newly developed, while correlations for the plate-fin heat sinks are selected from previous models. By using the appropriate correlations, thermal resistances of the optimized plate-fin and pin-fin heat sinks are compared under fixed pumping power conditions. Finally, a contour map, which depicts the ratio of the thermal resistances of the optimized plate-fin and pin-fin heat sinks as a function of dimensionless pumping power and dimensionless length, is presented. The contour map indicates that optimized plate-fin heat sinks possess lower thermal resistances than optimized pin-fin heat sinks when dimensionless pumping power is small and the dimensionless length of heat sinks is large. On the contrary, the optimized pin-fin heat sinks have smaller thermal resistances when dimensionless pumping power is large and the dimensionless length of heat sinks is small.     

Effects of Structured Roughness on Fluid Flow at the Microscale Level

It has been well established that there are no differences between microscale and macroscale flows of incompressible liquids. However, surface roughness has been known to impact the transport phenomena. This work aims to systematically quantify the effect of structured roughness geometries on friction factor in the laminar and turbulent flows as a precursor to the detailed heat transfer studies on these geometries. Experiments were conducted by varying the pitch (150–400 μm) and height (36–131 μm) of transverse rib roughness structures in rectangular channels such that the pitch-to-height ratio ranged from 2 to 8. The channel width was fixed at 12.70 mm and the length at 152.4 mm, while the channel gap was varied (230–937 μm). Tests were conducted over a Reynolds number range of 5–3400. The results are compared to the existing models, which do not account for specific roughness features such as pitch and height. A theoretical model is developed to predict the effect of roughness pitch and height on pressure drop along the channel length. Validation of the proposed theory is carried out by comparing the predictions with the experimental results. The model and the experimental results provide an understanding of the effect of two-dimensional structured roughness on the frictional losses in fully developed laminar flow.     

两流非对称中间包不同湍流控制器流场的物理模拟研究

通过水模型实验和数值模拟,研究了不同控流装置下两流非对称中间包内流体流动特性.结果表明:圆形湍流控制器与单挡墙组成的控流装置中间包液面波动大,容易发生卷渣现象;非对称长方形湍流控制器和高低挡墙组成控流装置的中间包,两流之间的平均停留时间差异是圆形湍流控制器和单挡墙组成控流装置中间包的1/3,且近长水口侧出口的平均停留时...     

Unsteady Convective Heat Transfer of Power-Law Fluid with Variable Fluid Properties in a Concentric Annulus Originating from a Polymer Flooding Process

We study the unsteady convective heat transfer of power-law fluid with variable fluid properties in a concentric annulus with isothermal surface. The problem is originated from the polymer flooding process between a sucker rod and oil well. A new power-law rheological model is proposed, which takes the effects of temperature on fluid viscosity and thermal conductivity into account. Numerical solutions are presented for velocity and temperature fields using the Chebyshev spectral method coupled with the strong stability-preserving Runge–Kutta time discretization. The exponential convergence is verified by accuracy testing between a smooth exact solution of the Partial Differential Equations (PDEs) with source terms and the numerical approximation of manufactured solutions. It is found that heat transfer is enhanced in the variable power-law index model, and a decrease in power-law index of pseudoplastic fluids promotes heat transfer due to the increased Nusselt number. Moreover, the influences of other parameters on convective heat transfer behaviors are discussed in detail.     

碳酸钠改性对凹凸棒石理化性能的影响

为提高低品位凹凸棒石的附加值,利用不同浓度碳酸钠对提纯后的低品位凹凸棒石进行改性处理,考察碳酸钠改性对其黏度的影响,并通过阳离子交换容量测定、Zeta电位、FT-IR、XRD、XRF、SEM及N₂吸附-脱附等手段研究了碳酸钠改性前后凹凸棒石的物理、化学性能变化。结果表明,碳酸钠浓度为0.225wt%时,改性凹凸棒石表现出最高的黏度,碳酸钠与Mg²⁺、Al³⁺、Ca²⁺ 的离子交换作用以及表面电荷变化是黏度提高的主要因素。     

Entropy Generation Due to the Flow of a Non-Newtonian Fluid with Variable Viscosity in a Circular Pipe

The non-Newtonian fluid can be considered as a third-grade fluid with variable viscosity. In this case, the rate of fluid strain can be formulated using the third-grade fluid analogy. In the present study, entropy generation due to non-Newtonian fluid flow in a pipe is investigated. A third-grade fluid with variable viscosity is accommodated in the analysis. Analytical solutions for velocity and temperature distributions are presented, and an entropy generation number is computed for different non-Newtonian parameters, viscosity parameters, and Brinkman numbers. It is found that increasing the non-Newtonian parameter lowers the entropy generation number. This is more pronounced in the region close to the pipe wall. Increasing the viscosity parameter and Brinkman number enhances the entropy generation number, particularly in the vicinity of the pipe wall.     

液力偶合器流场仿真软件开发及试验验证

以SRAR-CCM+为后台软件,二次开发液力偶合器流场仿真专用软件,提高了液力偶合器流场分析效率。进行了JO65XR液力偶合器输入转速500～1500(r.min-1),滑差为1.0%、1.5%、1.7%、2.0%、2.5%、3.0%、5.0%共51种工况的流场仿真。试验验证表明:两者吻合较好,平均仿真误差为8.79%,表明该专用软件具有较好的实用性和可靠性。     

镍基高温合金热物性参数计算及测量

利用热力学软件Thermo-Calc描述了某种镍基高温合金在不同温度下共存相的平衡摩尔分数以及体积分数,计算了密度、比热、线膨胀系数等合金热物性参数,并与实验测量值进行比较,计算值与实验测量值符合的很好。